Generally, it has been well known in the art that each of primary winding and secondary winding (coils) of a dry type transformer is treated with solid type cast material in order to provide a dry type transformer in which dielectric liquid or gas is used as an electrical insulating medium and for diffusing heat created in the windings (coils) or in the transformer core.
In addition, an oil-filled transformer using liquid such as oil as dielectric material as an electrical insulating medium and for diffusing heat created in the windings (coils) or in the transformer core, classified as above-ground-type, overhead-type and underground-type, etc. Here, a dry type transformer using gas such as circulating air as dielectric material is classified as ground-type and underground-type, which generally raise drawbacks of not having resistance against natural chemical reaction when exposed to ground or underground surrounding.
The manufacturing of the solid type or dry type transformer has been succeeded but is limited to transformer with relatively low power. Furthermore, the prior art of solid type or dry type transformer or a method of manufacturing of the same has some problems as follows.
First, in case of the prior art of the solid type or dry type transformer the thermal diffusion through solid type dielectric material is limited and the accumulated heat inside windings can produced hot spots or high thermal gradients that can lead to cracks and produce electrical arcs in the solid insulation system. In particular, in some cases, the breakdown of the solid insulation of the transformer may lead to transformer failure to operate properly. The cracks produced in the transformer can make the transformer mechanically unstable (breakage of transformer coils) which can lead to further break down of the dielectric media between coils and within core or within coils. The electric arc produced within the solid insulation material weakens the dielectric strength of the solid type insulation material, causing break down of the solid insulation system leading to severe damages or even to the explosion of the transformer. In addition, oil-filled transformer using transformer oil as dielectric insulation material, can caused environmental contamination when oil leaks out of the transformer tank when the tank is damaged due to corrosion or when the tank is ruptured due to the transformer failure.
A prior art manufacturing method of an above-ground dry type transformer requires a protective enclosure which is electrically connected to ground in order to eliminate risks of electric shocks to human beings or to animals wherein the protective enclosure is made of metal, such as steel, and large enough to cover the active part of the transformer (core and coils) and to provide enough electrical clearance between the active part and the grounded enclosure. The drawback of such a transformer arrangement is a large space requirement for installation which makes it difficult to install in small space. Additionally, the expansion and contraction due to the temperature variations within the coil cause mechanical stresses to the transformer coils.
Furthermore, in the prior art of dry-type transformer, when a large transformer, such as a power distribution transformer, is manufactured using a dielectric cast resin material, it is difficult to cure the cast resin material uniformly in order to provide uniform physical and dielectric properties to the overall body of the transformer.
Underground Oil-filled type, solid type or dry type transformer has been proposed in order to solve the aforementioned drawbacks of the above-ground type transformer, however, it has not solved the corrosion problem on its surface causing the oil leakage and etc. when it has been buried and operated in underground for a long time.
FIG. 1 shows process order of prior manufacturing method of a transformer. As shown in FIG. 1, it includes low voltage winding process (S10), high voltage winding process (S20), core and coils assembling process (S30), frame assembling process (S40), final assembling process (S50). However, according to the prior manufacturing method, it is difficult to manufacture solid type insulation, which may overcome drawbacks of solid type or dry type transformer.